Thermal flowmeters capable of measuring mass flow directly are now widely used as air flow meters for detecting the air intake amount of the internal combustion engine of an automobile or the like.
In recent years, thermal flowmeters have been proposed in which a sensor element is formed on a semiconductor substrate (e.g., a silicon substrate) by micromachining. Such semiconductor sensor elements are fabricated by forming a rectangular hollow space through a semiconductor substrate, depositing an insulating film of several micrometers to cover the hollow space, and placing a heat-generating resistor on the insulating film. Typically, temperature-sensing resistors are arranged adjacent to the heat-generating resistor, whereby the flow rate of gas flowing over the heat-generating resistor is measured based on the amount of heat transfer from the gas to the temperature-sensing resistors. The size of the heat-generating resistor is quite small (e.g., several hundreds of micrometers wide) and shaped into a film of several micrometers; thus, it is small in thermal capacity, highly responsive, and requires less power consumption.
On the other hand, various improvements are being made to increase the reliability of such sensors (e.g., by preventing damage to the sensors resulting from oil and dust particles present in the air passageways in which the sensors are installed). In one such improvement, to reduce contamination of a sensor element, the sensor element is installed within an air passageway that allows the passage of part of an air flow. This air passageway is bent into an irregular shape so that the effects of inertia can prevent oil or dust particles from colliding with the sensor element.
However, when a sensor element is installed in such an irregular air passageway, the inertia exerted on an air flow in the passageway changes according to the rate of the air flow. This in turn changes the direction of the air flow over the sensor element according to the rate of the air flow, resulting in an error in the flow rate detected by the senor element.
One of the reasons for such a detection error is that the temperature distribution near the heat-generating resistor becomes uneven due to the irregularity of the structures near the heat-generating resistor. Thus, even a small change in air flow direction will change the detection sensitivity of the sensor element. If the heat-generating resistor is further reduced in size, the temperature distribution near the heat-generating resistor becomes more uneven, resulting in larger measurement errors. To solve the above issue, the air flow meter of Patent Document 1 below is designed to adjust the amount of heat generated by a heat-generating resistor by locally increasing or decreasing the line width of the resistor so that a uniform temperature distribution can be achieved.